Whilst popular speakers are booked 1,2 sometimes 3 years in advance, they are rarely so busy speaking EVERY DAY that they would be unavailable to support a ‘lastminute.com” request.
Personally, I frequently take speaker bookings 12-18months ahead, however, rarely accept more than 2 speaking engagements a week. (allowing for travel and preparation time for each engagement). This therefore, does allow the opportunity to ‘help out’ and support lastminute.com requests as and when I am able.
The best thing is always to call and ask, the best that can happen is you get the speaker you are seeking and the entire event goes ahead without a hitch. The worst that could happen, is they are unable to help on this occasion, but offer you a string of other colleagues within the profession to try, many of whom you may not even be aware of!